Plasma Intrusion Process to Produce Cermet Armor

ABSTRACT

A plasma process producing cermets of differential hardness using nitrogen as the working gas and argon as the shielding gas at plasma temperatures is disclosed. Nitrogen atoms are ionized and react readily with molten titanium atoms of a substrate layer which then solidify during cooling to form TiN particles in the surface of a substrate thereby forming a composite. Because the nitrogen chemically bonds to the molten titanium the composite is not a laminate but an alloy of differential composition. By manipulating the process variables, the differential composition can be meticulously controlled. The thickness of the TiN cermet layer, and the hardness of the TiN cermet layer can be independently controlled in a reproducible manner thereby producing materials possessing specific desirable properties tailored to the ultimate intended use of the composites.

CROSS REFERENCE TO RELATED APPLICATIONS

None

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

None

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK

None

BACKGROUND OF THE INVENTION

This invention relates to high impact resistant and high wear resistant materials. More particularly, this invention relates to a computer-controlled process for forming required thickness and hardness-value composites of refractory (titanium, zirconium, hafnium, vanadium, niobium and aluminum) elements and alloys by controlling molten metal reactions with mixtures of reactant gases to form composites, cermets, of desired hardness as required by the identified application. No fibers, particles or secondary solid, or liquid materials are used.

The use of metallic based materials in the armor field have the advantage of multiple hit capability, visual assessment of damage, and possible on-site repair. In addition, metallic materials will be more readily attached to vehicles and damage will be visible and often repairable. Non-repairable material in the field will be recyclable and not disposed of as are ceramics and organics.

BRIEF SUMMARY OF THE INVENTION

A plasma intrusion process is computer controlled to produce controlled concentrations of refractory elements that nucleate and grow cermet materials is disclosed. The ability to control the concentrations of refractory elements at required depths in the base material or substrate enable the cermet to be used for various applications including ballistic protection, wear and erosion protection and high temperature impact and erosion protection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows impact data for substrates treated with the present invention and substrates not treated.

DETAILED DESCRIPTION OF THE INVENTION

A computer controlled plasma-intrusion process is used to produce a refractory-elemental and alloy composition in the surface region of refractory or other elements or alloys. Cermets produced according to the invention are highly impact resistant and well suited to ballistic armor. Additionally the materials produced according to the invention are extremely wear and abrasive resistant.

FIG. 1 compares penetration resistance for the instant invention with titanium armor and low density homogeneous (RHA) armor for a .30 cal. bullet. The two curves represent depths that can withstand impacts without penetration for one-half inch plate armor. Armor density is located on the horizontal axis and bullet velocity is plotted on the vertical axis. Armor according to the invention withstood velocities from 2873 ft/sec (point A) to over 3000 ft/sec (point B) as compared to a little more than 2500 ft/sec for titanium and 2000 ft/sec for RHA, at comparable densities.

Materials produced using the disclosed computer controlled method have controlled surface compositions of refractory elements or other elements or alloys. The materials comprise a “bottom” or substrate continuous region of a chosen refractory compound, or alloy, such as titanium or a titanium alloy, for example 6 A\4V alloy. Overlying the bottom or substrate layer is a processed or hardened “top” or surface layer comprising a continuous region including nucleated and grown refractory particles within the surface of the substrate layer, for example titanium nitride or titanium carbide particles in the titanium alloy, such as 6 A\4V. The top layer identifies refractory or alloy components that are processed to an application required Vickers hardness value between 500 to 1800 and a top layer thickness between 0.010 and 0.750 inches. A computer controlled plasma intrusion process is utilized to control the top layer thickness. A top layer produced according to the invention is formed and is the product of nucleation and growth of a second phase of TiN particles in the Ti alloy substrate. Due to the high temperature of the plasma intrusion process the top layer is formed in a molten chemistry process and the top layer is not a coating. As such the top layer does not separate or peal from the bottom substrate.

The resulting hardness and thickness of the top layer is controlled by the process variables. The initial starting substrate layer, for example titanium alloy Ti6\A4 will have a Vickers hardness of approximately 400 (Hv=400). The resultant processed composite product will have a desired upper thickness and hardness overlying an unprocessed substrate of desired thickness and hardness.

In a preferred embodiment of the material of the present invention, the cermet is formed to a required depth and surface concentration in a titanium plate. The titanium plate is processed to a required hardness value by quantitatively controlling the reactant gases and other variables to meet the product requirements. Typical variables include (a) the thickness of the underlying substrate layer, (b) the desired thickness of the top or processed layer, (c) the plasma torch distance above the bottom substrate layer, (d) the plasma torch travel velocity, (e) the plasma torch current (power level), (f) the height of the work piece (distance of the substrate lower layer from the plasma torch), (g) the working gas composition, (h) the working gas velocity, (i) the shielding gas composition, and (j) the shielding gas velocity.

A commercially available plasma welding torch or transfer arc is mounted vertically (90 degrees to the substrate surface) on a gantry apparatus to provide three (x,y,z) triennial control. A commercially available DMC-1700 Motion Controller, produced by Galis Motion Control, Inc. (2760 Atherton Road, Rocklin, Calif.) was utilized to control the plasma torch. The entire process including both the gantry apparatus and plasma torch are controlled using a standard laboratory or office computer.

To optimize the final properties of a desired product, whether it is for ballistic or wear resistant properties, the development of the processing procedure is the same.

During a normal plasma welding situation, argon would be used as the working gas as well as the shielding gas. Applicant has discovered that when using nitrogen as the working gas and argon as the shielding gas at plasma temperatures nitrogen atoms are ionized and react readily with molten titanium atoms which then solidify during cooling to form TiN particles in the surface of the substrate thereby forming a composite. Because the nitrogen chemically bonds to the molten titanium the composite is not a laminate but an alloy of differential composition. By manipulating the process variables, the differential composition can be meticulously controlled. The thickness of the TiN cermet layer, and the hardness of the TiN cermet layer can be independently controlled in a reproducible manner thereby producing materials possessing specific desirable properties tailored to the ultimate intended use of the composites. Additionally, the rate by which the upper layer changes in hardness can be controlled by adjusting the differential composition, or how much the chemical composition of the surface layer changes with depth.

Titanium has a Vickers hardness of approximately 400 (Ti—440 HV) while titanium nitride has a hardness of approximately 2200 (TiN—2200 HV). The concentration of TiN particles in the solidified base or substrate metal determines the hardness of the composite surface.

For a given application, the first step is to determine the required thickness of the surface layer that will optimize the composite for a given application. After this determination, a sample of base or substrate material is subjected to a single pass of constant velocity of the plasma torch. The plasma torch is optimally passed across the substrate at an angle of 90 degrees, maximizing the incident radiation on the surface of the substrate. The plasma torch is traversed across the surface of the substrate at a uniform, controlled velocity and height.

The surface of the sample is analyzed after the first pass is completed and the substrate cooled. If the surface is fairly smooth a narrow strip is cut from the base material and then cut in half in order to determine the depth of the material formed. Generally the upper composite layer will be darker than the base substrate. If the depth of the deposited material is satisfactory, a second cut is made to polish and mount in plastic for subsequent determination of the surface hardness. If the hardness profile of the deposited material is satisfactory, the first processing step is repeated until the required hardness and layer depths are fully realized.

With a satisfactory single pass result, process between three-to-five passes, overlapping the passes to obtain a minimum processed depth between the passes. Depending upon the proposed use, the passes could have no overlap to 50 percent overlap. The passes should be in alternating directions across the substrate and be made nearly to the edge of the substrate to minimize residual stress in the finished substrate to minimize warping. 

1. The process of producing a composite material of differential composition comprising ionizing nitrogen gas and reacting the ionized nitrogen gas with molten titanium atoms on a surface of a titanium substrate layer which then solidify during cooling to form a cermet layer of TiN particles in the surface of the titanium substrate thereby forming a cermet composite layer.
 2. The process of claim 1 wherein the nitrogen gas is ionized by passing the substrate through a plasma torch using a nitrogen working gas and argon as a shielding gas.
 3. The process of claim 1 including the steps of manipulating the thickness of the TiN cermet layer, and the hardness of the TiN cermet layer thereby producing materials possessing specific desirable properties tailored to the ultimate intended use of the composites by controlling the duration with which the titanium substrate is subjected to the plasma torch in the nitrogen atmosphere.
 4. The process of claim 1 including the steps of manipulating the thickness of the TiN cermet layer, and the hardness of the TiN cermet layer thereby producing materials possessing specific desirable properties tailored to the ultimate intended use of the composites by controlling the number of times with which the titanium substrate is subjected to the plasma torch in the nitrogen atmosphere.
 5. A composite material of differential composition produced by the process of claim
 1. 